Radiation-sensitive elements and their preparation



United States Patent 3,178,293 IATION-SENSiTlVE ELEMENTS AND "mam PREPARATION John Howard Bigelow, Rochester, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del, a corporation of Delaware No Drawing. Filed Get. 25, 1962, Ser. No. 233,148 5 Claims. (Cl. 95-168) This invention relates to radiation-sensitive elements and, more particularly, to light-developable, direct writing, radiation-sensitive elements having improved sensitivity to a high intensity beam of light or other electromagnetic radiation and to their preparation.

Radiation-sensitive elements adapted for light-record ing, e.g., oscillographic recording, are known. Such materials comprise both the developing-out type and the printout type. The developing-out type, as the name implies, requires that the photographically exposed material be chemically developed, fixed and washed in order to produce a visible image which is stable against fading or other changes which in time detract from the quality of the recorded image.

The print-out material requires no chemical development step to produce a visible image and may or may not be fixed to provide an image which is not subject to fading or other deterioration. Such materials are generally slower than those materials used in developing-out processes and have poor image permanency.

A third type of radiation-sensitive material especially suitable for light printing and oscillographic recording comprises a silver halide emulsion layer which, when exposed to a high intensity source of radiation forms a latent image which can then be developed by exposure to difiuse daylight or artifical light of lower intensity. Such materials are faster than the print-out materials described above and require no chemical processing; and, therefore, they afford considerable advantage in doing away with liquid solutions and the necessary processing equipment. Because of image fading and background instability under extended periods of exposure to ambient light, it is desirable to process the image chemically for permanent retention thereof. It is this third type of printing material with which the present invention is concerned.

The prior art suggests for this third type of direct-printing material, silver bromide emulsions which have incorporated therein, sulfur-containing compounds such as halogen acceptors, e.g., thiourea, thiosemicarbazides, etc. The background and image densities of layers comprising such emulsions, howevenare not sufficiently stable to'permit rapid access to, or prolonged examination of, the recorded images, particularly under conditions of daylight or normal room lighting. It has also been proposed to make photosensitive emulsions for direct-printing or writing by introducing silver thiocyanate into a silverbromide emulsion. The sensitivity of this material is alleged to be considerably greater than pure silver bromide emulsions containing halogen acceptors of the above type and can be further increased by the use of such compounds. Stability of image and background is also a problem with this latter material. Upon prolonged exposure to ordinary illumination, the background darkens and the image .regresses, depending on the exposure conditions, which reduces the ratio between background and image densities. There may also be conditions under which both image and background densities are reduced. Also, Where the development radiation contains a high proportion of ultra- .violet light, subsequent exposure after light development .causes an increase in background density and rapid regression of the light-developed image. It is necessary to resort to bathing the light-developed image in a liquid developing "ice bath containing a silver halide solvent to intensify the image density and preserve or increase the contrast or ratio of background to image density of the originally lightdeveloped image, Treatment in a fixing bath is also recommended for the same purpose. V

Improved types of light-developable, direct writing radiation-sensitive elements have been suggested in assignees Hunt US. Patents 3,033,678 and 3,033,682. Both of these patents disclose and claim elements comprising silver halide emulsions having an average grain size in the range 0.1-10 microns and containing from 0.5 to 120 mole percent of a stannous salt based on the silver halide and preferably 5 to 40 mole percent. Such elements have greatly increased writing speeds and more readily stabilized background densities. In the Hunt Patent 3,033,678 the background is stabilized by heating the element after image exposure to a temperature of between 250 F. and 280 F. before or during part of th light development step. The stannous salt is added after the precipitation and ripening operations. An excess of bromide ion over that necessary to react with silver nitrate is desirable. In the Hunt Patent No. 3,033,682 a lightdevelopable, direct-writing light-sensitive element is disclosed and claimed wherein the light-sensitive emulsion comprises an organic colloid-silver halide emulsion containing 0.5 to 120 or more and preferably 5 to mole percent of a stannous salt, 0.1 to 20 or more, and preferably O.5 to 5 mole percent, of a plumbous salt and an excess of halide ions, all per mole of silver. The presence of the plumbous salt serves to stabilize the background and prevent a reduction in the ratio of image to background densities. The stannous and plumbous salts are added after the emulsion is precipitated and ripened to provide a grain size of from 0.1 to 10 microns.

While the photosensitive elements of the above Hunt patents otter many improvements in terms of higher writing speed, image and background stability they are not entirely satisfactory for a number of reasons. For example, although their background stability is far superior to other known direct writing elements and will retain a readable record over a period of several hours under the illumination of normal room lighting, background density will in time develop to a point where the image record is not sufficiently clear. 'Ihe image density also has a tendency to regresswhich is undesirable. These changes occur over periods of days of exposure to normal illumnination. In the past, it has been the practice of operators to wet process these direct writing elements Where it is desirable to keep a permanent record and to use the record to make duplicate records by photographic processes. To develop direct writing elementsof the above type it requires very special developing compositions containing a large amount of alkali halide to restrain fog and a silver halide solvent. This is of course more inconvenient than using conventional developers. 7

A further disadvantage of the elements is their instability, when light-developed only, to ultraviolet radiation or other radiation more intense than normal room lighting as would be used in exposing duplicating material i.e. azo copies, etc. as used in the conventional ofiice copying equipment. This is a very serious disadvantage because it does not permit reproducing the original records without deterioration of said records. A'further disadvantage is the fact that the photosensitive elements have a tendency to lose'speed and density uponprolonged storage.

An object of this invention is to provide new and improved direct writing, radiation-sensitive photographic emulsIons, emulsion layers and elements, e.g., papers. another object is to produce a direct-writing photosensi t ve element having improved sensitometric characteristlcs in terms of higher speed and maximum density. A further object is to provide a direct writing photosensitive element which, after light development, is sufficiently stable to allow reproduction by photographic copying means. A still further object is to provide a direct-writing photosensitive element having improved aging stability and consequently increased shelf life. A still further object is to provide a direct-writing photosensitive element which may be developed in conventional developers that are commercially available.

The process of making light-developable, direct-writing silver halide emulsions of this invention comprises (1) admixing aqueous silver .nitrate and from 0.33 to 10 mole percent, based on the silver, of a water-soluble plumbous salt (0.0033 to 0.1 mole of plumbous salt per mole of silver), with an acidified aqueous solution of a Water-soluble halide containing a water-permeable organic colloid binding agent; (2) ripening the precipitated silver halide emulsion, (3) digesting the resulting emulsion, and prior to completion of digestion or coating, incorporating with the silver halide emulsion, based on the silver,

(a) to 120 mole percent of a water-soluble inorganic bromide,

(b) a water-soluble plumbous salt in an amount of 0 to 5 mole percent, and one of the following:

(0) 5 to 50 mole percent of a water-soluble stannous salt, and

(d) to 350 mole percent of sodium nitrite.

The silver halide and plumbous salt are added so that the plumbous salt is present at the time of precipitation of the silver halide and preferably by the use of an aqueous solution containing both of the components. The precipitation is preferably carried out using potassium chloride and/or potassium bromide as the water-soluble halide salts. The precipitation is carried out by slowly adding an aqueous solution of the silver nitrate and plumbous salt to an acidified aqueous solution of a water soluble halide salt containing a water-soluble dispersing agent, e.g., gelatin. In the case of silver bromide gelatin emulsions it has been found advantageous to add the soluble bromide salts in an amount sutficient to provide a considerable excess of bromide ion over that which is necessary to react with the silver nitrate. The important thing, however, is that the lead ion be part of or incorporated in the silver halide crystal. A variety of soluble salts, e.g. potassium, sodium and ammonium chloride and bromide salts can be used.

After precipitation and ripening the emulsion may or may notbe, but preferably is, washed in the manner described in assignees Moede, U.S. Patent 2,772,165. The emulsionis redispersed and digested in the conventional manner. At this point or prior to digestion, optical sensitizing dyes may be optionally added to increase the spectral response of the emulsion layer for use in instruments employing a variety of, light sources. During this operation from 0 to 5 mole percent of a soluble plumbous salt, from 0 to 120 mole percent of a soluble bromide salt and either 5 to mole percent of a soluble-stannous salt or 30 to 350 mole percent of sodium nitrite, all based on the silver halide are added. After the digestion step, the usual coating additions, e.g., hardener, wetting agents, etc. are added and the viscosity is adjusted as desired by the addition of a further amount of gelatin or other colloid. In general the ratio of gelatin to silver halide is 2:1, however this is not at all critical. The prepared emulsion is then coated on a suitable support, e.g., paper and dried to give a dry coating weight equivalent to about 30 mg.

AgBr/drnF.

To determine the sensitometric characteristics of the material it may be exposed through a power of 2 step wedge in an electronic flash sensitometer similar to that described by Wyckofr' and Edgerton, Journal of the Society of Motion Picture and Television Engineers, 66, 474 (1957). This instrument uses a xenon discharge tube as the source of radiation and has available two exposure times of 10 and 1000 microseconds. Relative sensitivities of materials measured with this instrument can be expressed as the number of steps recorded in the image. The exposed material may be light-developed by irradiation by exposure to room lighting or with light from a fluorescent black light tube at about meter-candles intensity although the intensity of this radiation is not critical.

' The images became easily visible in approximately 0.1

to 15 seconds although longer times may be used. To determine the densities of the image and background, a reflection densitometer may be used whose values correspond to visual density. To test the stability of the background the light-developed image record is exposed to room light at 50 meter-candles continuously for 16 hours. Speed in oscillography is measured in inches per second and is called writing speed. The radiation source in a typical instrument designed for the above direct-writing papers is the Osram super high pressure are lamp Type HBO 107/ 1. Writing speeds are determined from the frequency of the signal and the peak to peak amplitude of oscillation as recorded on the paper.

In order to more specifically illustrate the features of this invention, the following examples. are set forth below. They are not intended to limit the scope of the invention except as set forth in the claims.

Example I A gelatino-silver chlorobromide emulsion was made by slowly adding an aqueous solution containing a mixture of 1 mole of silver nitrate and 0.033 mole of lead nitrate, Pb(NO to a gelatin solution containing 1 mole of potassium chloride and acidified with .05 mole of hy drochloric acid. The precipitation was carried out under a Wratten Series 1 red safelight manufactured by the Eastman Kodak Company. The temperature at precipitation and for 40 minutes thereafter was held at 140 F. After precipitation, an aqueous solution of 1.6 moles of potassium bromide was added while the mixture was held at 140 F. The resulting emulsion was coagulated, washed and redispersed in a manner similar to that described in assignees Moede US. Patent 2,772,165. A mixture of the redispersed emulsion, gelatin necessary to provide about 9% concentration for coating, 0.02 mole of lead nitrate, 0.6 mole potassium bromide, and 0.2 mole of stannous chloride (SnCl per mole of silver bromide was digested for 20 minutes at F. After digestion, the mixture was cooled to coating temperature, borax was added as a buffer, coating aids and chrome alum were added, and after adjusting the pH to 4.0 and adjusting to suitable coating viscosity the emulsion was coated on a paper support to give a dry coating Weight equivalent to 30 mg. of silver bromide per square decimeter. The coated emulsion was dried in a conventional manner. A sample of the coated material when exposed in an oscillograph as described above gave a Writing speed of 23,550 inches per second. Another sample of the coated material was given an exposure of 1000 microseconds in the flash tube sensitometer described above using a power of 2 step wedge. The exposed element was light-developed for 15 minutes by radiation with light from a fluorescent black light tube at 75 meter-candle intensity. Twelve steps became visible and the image had a maximum density of 0.95 (maximum image density of 0.51 plus background density of 0.44) and was quite stable. When exposed to room light at 50 meter-candles intensity for 16 hours the maximum density dropped to 0.86 but the contrast was improved because the image density only dropped to 0.49 while the backgrounddensity dropped to 0.37. The image was stableenough to allow reproduction by exposing on office copying equipment using tungsten or mercury vapor light sources without image deterioration. The unexposed element showed good aging stabilty or shelf lfe.

Example 11 7 Example I was repeated up to the point of redispersing the coagulated and washed emulsion. After redispersion, gelatin, 0.01 mole of plumbous nitrate, 0.3/mole potassium bromide, and 1.67 moles of sodium nitrite, all per mole of silver bromide were added and the mixture was digested for minutes at 130 F. After digestion, this mixture was cooled, and borax was added as a bufier, coating aids and hardener were added, the pH was adjusted to 8.5 and the resulting emulsion was coated on a paper support and dried. The material had a writing speed approximately the same as that of Example I.

The material was wet processed by immersing it for 1 /2 minutes in a developing composition at 68 F. having the following formula:

Grams Metol 1.5 Hydroquinone 6.0 Sodium sulfite (anhydrous) 19.5 Sodium carbonate (anhydrous) 28.0 Potassium bromide 0.8

Water to make 1.0 liter.

The developed element was rinsed and then immersed for 5 minutes in a fixing solution having the following composition:

Sodium thiosulfate grams 240 Sodium sulfite (anhydrous) do 15 Acetic acid, 28% ml 47 Potassium aluminum sulfate grams 15 Water to make 1 liter.

The resulting image had a stable image density of 1.3 and a stable background density of 0.00. The processed element was quite suitable as a permanent record and could be reproduced by conventional office copy equipment. The material could also be exposed and light developed in the manner described in Example I to give quick access to the recorded images.

Example 111 Example IV Example II was repeated in which 13.5% of dextran based on the weight of gelatin was added at digestion. Under identical test conditions it was found that the writing speed was the same as in Example 11 but that the maximum density in the developed image was increased slightly.

Example V Example I was repeated except that 0.0033 mole of i Pb(NO was used instead of 0.033 mole in the AgNO solution in precipitating the emulsion. When this coated paper was exposed on a flash tube sensitometer, photolyzed and the density observed as in Example I, 12 steps were seen and the total density read 0.78 (D-max.=0.36, background=0.42).

Example VI Example I was repeated except that 0.1 mole of Pb(NO was used instead of 0.033 mole in the AgNO solution in precipitating the emulsion and 0.3 mol KBr instead of 0.6 mole at digestion. When this coated paper was exposed on a flash tube sensitometer, photolyzed and the density observed as in Example I, 10 steps were seen and the total density backgroimd==03 8).

Example VII coated paper was exposed on a flash tube sensitometer,

v photolyzed and the density observed as in Example I, 12 steps were seen and the total density read 0.97 (D-max.=0.46, background=0.51).

Example VIII 1 Example I was repeated except that 0.05 mole of Pb(NO and 0.3 mol KBr were added at digestion instead of 0.01 and 0.6 mol respectively. When this coated paper is exposed on a flash tube sensitometer, photolyzed and the density observed as in Example I, 12 steps can be seen and the total density read 0.97 (D-max.=0.54, background=0.43

Example IX Example I was repeated except that 'no Pb (NO and 0.3 mol KBr were added at digestion instead of 0.01 and 0.6 mole, respectively. When this coated paper was exposed on a flash tube sensitorneter, photolyzed and the density observed as in Example I, 12 steps were seen and the total density read 0.92 (D-max.:'0.47, background: 0.45

Example X Example I was repeated except that 0.05 -mole Pb(N0 0.3 mole KBr, and 0.4 mole SnCl were added at digestion instead of 0.01, 0.6 and 0.2, respectively. When this coated paper was exposed on a flash tube sensitometer, photolyzed and the density observed as in Example I, 12 steps were seen and the total density read 0.89 (D-max.=0.53, background=0.36).

Stannous chloride may be added from aqueous solutions, particularly when such solutions are made using the anhydrous stannous compound The action on the emulsions of this invention by halogen acceptors is not affected by excess bromide ions and in this respect the range of concentration'ot bromide ion is not critical. This is of considerable advantage compared with the 1 prior art emulsions where sulfur compounds are used which are adversely affected by the presence of any substantial excess of bromide ions.

Where it is desired, other halides or combination of halides may be used to: form the silver halide grains. For example, pure silver chloride or pure chlorobromide may be used. Where soluble chloride salts are used it is desirable, because of solubility diiterences, to form the silver halide grains of des red composition and size and then add suficient soluble bromide salts to provide the desired concentration of bromide ions. The role of the lead salt at precipitation is not fully understood but it is believed that its presence in the silver halide crystal makes the internal latent image formed by the high intensity writing trace more easily available which is not the case when the lead salt is added at a later stage of the emulsion preparation. The addition of lead salts at a later stage appears to suppress the sensitivity in the unexposed areas of the emulsion layer.

In place of the gelatin binding agent used in the foregoing examples there can be substituted other material or synthetic Water-permeable organic colloid binding agents. Such agents include water-soluble or permeable polyvinyl alcohol and its derivatives, e.g., partially hydrolyzed polyvinyl acetates, polyvinyl ethers, and acetals containing a large number of extral-inear CH CHOH- groups; hydrolyzed interpolyrners of vinyl acetate and unsaturated addition polymerizable compounds, for example maleic anhydride, acrylic and methacrylic acid esters and styrene. Suitable colloids of the last mentioned type are disclosed in US. Patents 2,276,322; 2,276,-

read 0.85 (D-max.=0.47,

amazes.

323 and 2,397,866,. The useful polyvinyl acetals include polyvinyl acetaldehyde acetal, polyvinyl butyraldehyde acetal and polyvinyl sodium o-sulfobenzaldehyde acetal. Other usefiul colloid binding agents include the polyvinyl lactams of Bolton US. Patent 2,495,918 e.g., poly-N- vinylpyrrolidone; the hydrophilic copolymers of N-acrylamido a-lkyl betaines described in Shacklett U.S. Patent 2,833,650 and hydrophilic cellulose ethers and esters.

Suitable supports for the novel photographic emulsions of thisinvention includes those used in the prior art for light-writing and oscillographic recording. The preferred support is photographic grade paper but may be a hydrophobic film composed of a cellulose ester, e.g. cellulose acetate or a polymer,-e.g., the film supports disclosed in Alles et al. US. Patent 2,627,088 and Alles U.S.'Patent 2,779,684.

- The novel process of this invention produces lightdevelopable, direct-writing, photosensitive emulsion layers and elements having several advantages over the prior art products. The emulsion layers and elements upon exposure to high-intensity radiation and subsequent light development, yield images of higher maximum density and greater stability against image fading than are obtainable with direct writing elements known heretofore. In addition to being extremely convenient to use because wet processing can be eliminated, it is also adaptable to conventional chemical development where desired. The invention also provides an element wherein one may subject the exposed material to relatively high illumination for long periods of time or use the material to photographically reproduce the image record using high intensity exposing radiation without serious image deterioration. The elements of this invention have high photographic speed and permit rapid access to the recorded image. They also provide, images having higher maximum density and image stability than are obtainable with the prior art materials. The elements of this invention may also be processed in the manner disclosed in assignees Hunt patents referred to above and in Hunt applications U.S. Serial No. 137,534, filed September 12, 1961, and US. Serial No. 152,914, filed November 16, 1961. This invention also provides direct-writing photosensitive elements having improved aging stability.

In the case of the photosensitive elements made by addition of sodium nitrite to the precipitated silver halide (.3 emulsion, the invention has the important commercial advantage that conventional developer solutions free from restrainers, e.g., soluble bromides, can be used. Still further'advantages will be apparent from the foregoing de-.

scription of the invention.

What is claimed is:

l. A process of making light-developable, direct-writing silver halide emulsions, whichv comprises,

(1) admixing aqueous. silver nitrate and from 0.33 to 10 mole percent, .based on the, silver, of a watersoluble plumbous. salt (0.0033 to 0.1 mole of plumbous Saltper mole of silver), with an acidified aqueous solution of a water-soluble halide containing a water-permeable organic colloid binding agent;

(2) ripening the precipitated silver halide emulsion,

(3) digesting the resulting emulsion, and prior to completion of one of the steps of digesting and coating, incorporating with the silver halide emulsion, based on the silver,

(a) 0 to, 1.20 mole percent of a water-soluble inorganic bromide,

(b) a water-soluble plumbous salt in an amount of 0 to 5 mole percent, and one of the followmg:

(0) 5 to mole percent of a water-soluble stannous salt, and

(d) 30 to 350 mole percent of sodium nitrite.

2. A process accordingto claim 1 wherein the one constituent admixed .with any added inorganic bromide and plumbous salt is sodium nitrite.

3. A process according to. claim 1 wherein one constituent admixed with any added inorganic bromide and plumbous salt is a water-soluble stannous salt.

4. A process according to claim 1 wherein said emulsion is a gelatino-silver;halide emulsion.

5. A process according to claim 1 wherein said emulsion is a gelatino-silver chlorobromide emulsion.

References Cited by the Examiner UNITED STATES PATENTS 3,033,682 5/62 Hunt 96l08 NORMAN G. TORCHIN, Primary Examiner. 

1. A PROCESS OF MAKING LIGHT-DEVELOPABLE, DIRECT-WRITING SILVER HALIDE EMULSIONS WHICH COMPRISES (1) ADMIXING AQUEOUS SILVER NITRATE AND FROM 0.33 TO 10 MOLE PERCENT, BASED ON THE SILVER, OF A WATERSOLUBLE PLUMBOUS SALT (0.0033 TO 0.1 MOLE OF PLUMBOUS SALT PER MOLE OF SILVER), WITH AN ACIDIFIED AQUEOUS SOLUTION OF A WATER-SOLUBLE HALIDE CONTAINING A WATER-PERMEABLE ORGANIC COLLOID BINDING AGENT; (2) RIPENILNG THE PRECIPITATED SILVER HALIDE EMULSION, (3) DIGESTING THE RESULTING EMULSION, AND PRIOR TO COMPLETION OF ONE OF THE STEPS OF DIGESTING AND COATING, INCORPORATING WITH THE SILVER HALIDE EMULSION, BASED ON THE SILVER, (A) 0 TO 120 MOLE PERCENT OF A WATER-SOLUBLE INORGANIC BROMIDE, (B) A WATER-SOLUBLE LPLUMBOUS SALT IN AN AMOUNT OF 0 T 5 MOLE PERCENT, AND ONE OF THE FOLLOWING: (C) 5 TO 50 MOLE PERCENT OF A WATER-SOLUBLE STANNOUS SALT, AND (D) 30 TO 350 MOLE PERCENT OF SODIUM NITRITE. 